1. Field of the Invention
This invention relates to a solid state laser having a wavelength-conversion function, and more particularly to a wavelength-conversion laser in which an etalon and a Brewster's plate are provided in a resonator to realize a single longitudinal mode oscillation.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a solid state laser wherein a solid state laser crystal doped with neodymium (Nd) is pumped with a laser beam emitted from a semiconductor laser or the like. In this type of a solid state laser, it is widely practiced to convert the laser beam emitted from the solid state laser to its second harmonic by a nonlinear optical crystal disposed in the resonator of the solid state laser in order to obtain a laser beam having a shorter wavelength.
Such a wavelength-conversion laser as well as other types of lasers has been required to oscillate in a single longitudinal mode in order to suppress output fluctuation resulting from longitudinal mode competition.
In order to meet this requirement, there has been developed a solid state laser in which a Nd:YVO.sub.4 crystal (a YVO.sub.4 crystal doped with Nd), which facilitates realization of oscillation in a single longitudinal mode, is employed as a solid laser medium, a crystal for realizing type-I phase matching is disposed in the resonator as a nonlinear optical crystal and an etalon is disposed in the resonator.
However in such a wavelength-conversion laser, utilization of anisotropy of YVO.sub.4 crystal (difference in gain between the n polarization and the .sigma. polarization) is not sufficient for polarization control and especially when the pumping power is high, .sigma. polarization oscillation occurs, which is apt to put the laser into a multiple longitudinal mode.